This invention relates to a method and a system for controlling airflow in a multiple bed desiccant drying system, particularly in a twin tower desiccant dehumidifier, and particularly during a regeneration phase, and during transition periods between phases.
Multiple desiccant bed systems used for drying a synthetic plastic material are known, in which a moisture-laden gas stream is formed as the exit gas from a hopper in which plastic granules are dried by a stream of drying air. During an adsorption phase, the exit gas is conducted through one or more drying vessels filled with an adsorption medium, whereby the adsorption medium extracts the moisture from the gas so that the resulting dry gas can be used again as a drying gas for drying plastic granules.
When the adsorption medium in a drying vessel is saturated with moisture, the drying vessel is transferred to a regeneration phase in which heated ambient air is conducted through the adsorption medium which takes up and carries away the moisture which was adsorbed therein. The ambient air used to dry the adsorption medium typically contains moisture, which increases the drying time required to regenerate the adsorption medium.
During the regeneration phase, since the adsorption medium is heated by the heated regeneration air, the adsorption medium is typically subsequently cooled with a cooling air flow prior to a transition to the adsorption phase. If moisture-laden ambient air is used as the cooling air, the adsorption medium will adsorb the moisture therefrom, reducing the efficiency (i.e. dryness) of the regeneration process.
By using a multiple bed system, the drying process can be continued essentially without interruption, by utilizing one or a portion of the beds for adsorption, while simultaneously regenerating other of said beds, by appropriately channeling the process air flow.
In commonly owned prior U.S. patent application Ser. No. 08/874,654 to Crawford et al., the disclosure of which is expressly incorporated by reference herein, a system and method of operation is disclosed in which two towers are connected by a 4-valve system. The 4-valve system is controlled such that the process air stream is progressively moved from the saturated bed to the regenerated bed. In that invention the terminal disruption of the dewpoint is minimized by any residual heat that remains in the fresh tower being brought online.
In previously known drying systems, a problematic issue is disruption of the process air temperature and dewpoint quality when changing from the saturated tower to the freshly regenerated tower. Furthermore, in previously known drying systems, bringing a heat exchanger online at the appropriate time without undue complexity of valves is a known problem. As an additional issue, previously known drying systems may be contaminated by room air during diverter valve changes. In addition, since the common instrumentation used to determine the dryness of the process air stream may require from 30 to 60 minutes to recover from the exposure to a high intermittent dewpoint, it is difficult to determine and monitor the humidity level of the process air in previously known drying systems.
In view of the above, there is a need for an improved method and a system for controlling airflow in a multiple bed desiccant drying system.
There is also a need for a method and a system which can be implemented in existing multiple bed desiccant drying systems with a minimum of components, effort, and cost.
These and other needs have been met according to the present invention as discussed in the following.
The present invention utilizes a multiple position operator for the pair of four way diverter valves which direct the process and regeneration air to the desiccant towers, in conjunction with two cutoff valves at the ambient connection ports for the regeneration air. This allows the desiccant cooling of the regenerating tower to utilize a slipstream of the dry process airflow, in order to avoid loading the regenerated bed with moist ambient air. The existing four way diverter valves are used to accomplish the diversion of the process air for the closed loop cooling of the desiccant by incorporating a multiple position actuator to seal the regeneration side of the dryer during the purge cycle, and then open slightly to allow some air to bypass through the regenerating desiccant tower for cooling. This avoids undue complication and expense.
The present invention limits the disruption of the process air dewpoint at tower change over. The present invention avoids the possibility of ambient room air contaminating the dry air circuit when open regeneration ports become connected to the dry air system while the tower diverter valves are in operation. Furthermore, by using a heat exchanger to remove some of the residual regeneration energy, the present invention avoids the use of ambient room air for desiccant cooling.
The present invention""s ability to seal off the desiccant beds from the ambient atmosphere has further advantages. By sealing the desiccant beds off from the ambient atmosphere, the desiccant can be maintained in usable (i.e., dry) form for extended periods of time, for example while waiting to be moved on-line, or even when the machine is shut down. This eliminates time delays during start-up, which are required in conventional desiccant drying systems to dry out the desiccant which is exposed to the atmosphere, and reduces energy requirements.
By introducing a multi ported valve in the regeneration airflow circuit, the present invention isolates the tower system from the ambient air that would otherwise enter the drying system while the diverter valves are in intermediate or indeterminate positions during tower exchange.
The present invention also allows the use of a regeneration cooling heat exchanger with only the multi port valve as the sole additional operating device. The multi port valve may have a number of mechanical arrangements, for example both directly operated poppets, and a combination of check valves and poppet valves.
The present invention may be either incorporated into new, or retrofitted to existing, twin tower desiccant dehumidifiers. The disruptive effects of changing the tower diverter valves of a conventional twin tower desiccant dehumidifier are substantially reduced. This is accomplished through the use of a multi port valve applied to the regeneration air circuit of the dehumidifier system.